7423

• UCREFRP Catalog Number: 7423
• Author: U.S. Bureau of Reclamation.
• Title: Glen Canyon Environmental Studies Draft Report, September 1987.
• USFW Year: 1987.
• USFW - Doc Type: Washington, D.C.
• Copyright Material: NO

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Recurrent flooding could therefore cause a severe re-
duction in areas of camping beaches and sand substrate
for vegetation in Grand Canyon. Loss of beaches is
most severe in the narrow reaches of the canyon where
camping beaches are already scarce. An example of how
beach deposits are inundated by flood releases is shown
in Figure VI-3. Photos of a beach deposit prior to and
after the 1983 flood releases are shown in Figure VI-4.

Loss of substrate will result in a loss of riparian
vegetation because the densest stands of vegetation
commonly occur on sand deposits near the water's edge.
Vegetation can also be destroyed by inundation,
removal, or burial by redeposited sand. Long-lived
terrestrial vegetation therefore cannot become
established below the level of the highest frequently
recurring flow. At floods of 90,000 cfs, up to 50
percent of the total plant cover may be lost in some
areas. Ninety-five percent of the marshes along the
river were lost during flooding in 1983-1986.
Scouring of marshes was so severe that they may not
recover. (See Appendix B, section v.)

Wildlife populations which use vegetation for resting,
nesting, and feeding will gradually decline in numbers
due to the loss of habitat area. Loss of bird
reproduction is especially acute if flooding occurs
during the spring nesting season. Mammals and reptiles
are affected through the drowning of individuals
during high flows as well as the gradual loss of num-
bers through habitat reduction (Appendix B, section v.)

White-water boating. Floods also have a negative
impact on white-water boating. Surveys of white-water
guides, NPS accident records, and observations of over
5,000 boats running rapids under different flows, show
that flood releases significantly increase both the
hazard associated with running rapids and the number of
boaters that choose to walk around difficult rapids.
For example, at Crystal Rapid, nearly 50 percent of
boats have passengers walk at flood flows, compared to
20 percent at flows between 10,000 and 31,500 cfs. The
chance of flipping a boat when running a major rapid
rises from 3 percent to 8 percent as flows rise from
high (16,000-31,500 cfs) to flood (31,500-50,000 cfs).